The first five episodes of Big Brother 2011 have averaged 1.7 million viewers. Photograph: Channel 5

The Big Brother effect appears to be wearing thin on Channel 5, with audiences for the first five episodes of the reality show down nearly 50% on the last series on Channel 4.

Richard Desmond's channel enjoyed a ratings surge on the back of Celebrity Big Brother, but the regular edition of the show which followed immediately after has failed to match its appeal.

The first five episodes have averaged 1.7 million viewers, an 8.8% share of the audience, according to overnight figures.

This compares with an average of 3.2 million viewers, a 14.6% share, for the opening five instalments of the final run of Big Brother on Channel 4 last year.

However, so far Big Brother's ratings for Channel 5 are more than 10% up on the slot average for the past three months.

The reality show is also still rating double the channel's 2010 average audience share of 4.4%. Bought for an estimated £200m in a two-year deal, Desmond will be hoping it gives his channel a significant ratings boost.

Last night's instalment of the Endemol-produced show drew 1.4 million viewers, an 8% share, between 10pm and 11pm.

Channel 5 has declined to specify exactly how long Big Brother will run in its nightly 10pm slot – moved to 9pm for eviction nights and other special editions – but it is likely to last until November.

Shows have a habit of shipping viewers when they switch to Channel 5 – Neighbours lost about 300,000 when it moved from BBC1 in 2008. The broadcaster has the lowest audience share of the five main terrestrial networks.

Last year's 11th series, buoyed by publicity that it was also the final one on Channel 4, enjoyed a lift to average 2.9 million.

All ratings are Barb overnight figures, including live and same day timeshifted (recorded) viewing, but excluding on demand, +1 or other – unless otherwise stated. Figures for BBC1, ITV1, Channel 4 and Channel 5 generally include ratings for their HD simulcast services, unless otherwise stated

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